1. Field of the Invention
The present invention relates to a constant voltage circuit used at a wide temperature range, such as a constant voltage circuit used in a car control circuit or the like.
2. Description of the Prior Art
FIG. 5 is a circuit diagram of a prior art constant voltage circuit for stabilizing a voltage from a DC power source. In FIG. 5, reference numeral 100 denotes a differential amplifier, 101, 102 and 103 fixed resistors having resistance values R11, R12 and R13, respectively, and 104 a Zener diode which is a temperature-dependent semiconductor element. In this constant voltage circuit, a series circuit formed on one side by connecting the fixed resistors 101 and 102 in series and a series circuit formed on the other side by connecting the fixed resistor 103 and the Zener diode 104 in series on the other side constitute a bridge circuit, a connection point between the fixed resistor 101 and the fixed resistor 103 of the bridge circuit is connected to the output terminal of the differential amplifier 100, and a connection point between the fixed resistor 102 and the Zener diode 104 is connected to the ground. Further, the output of the connection point of the series circuit on one side is applied to the inversion input terminal of the differential amplifier 100 and the output of the connection point of the series circuit on the other side is applied to the non-inversion input terminal of the differential amplifier 100 so as to output a constant voltage from the output terminal of the differential amplifier 100.
A description is subsequently given of the operation of the above constant voltage circuit.
The voltage V+ of the non-inversion input terminal of the differential amplifier 100 is equal to the voltage Vz at both ends of the Zener diode 104. Therefore, the voltage V_ of the inversion input terminal of the differential amplifier 100 becomes equal to V+ and Vz. Consequently, the output voltage V6 of the differential amplifier 100 is represented by the following equation (1). EQU V6={(R11+R12)/R12}.multidot.Vz (1)
The above output voltage V6 is a constant value determined by the resistance values R11 and R12 of the fixed resistors 101 and 102 and the Zener voltage Vz of the Zener diode 104. The output voltage V6 of the differential amplifier 100 is referred to as "constant voltage" hereinafter.
Since the Zener diode 104 is a temperature-dependent element, the voltage Vz at both ends of the Zener diode 104 is changed by temperature. The voltage Vz at both ends of the Zener diode 104 is determined by a current Iz running through the Zener diode 104 and changes .DELTA.Vz in the voltage Vz caused by a temperature variations (to be referred to as "temperature characteristics" hereinafter) are determined by the voltage Vz. That is, the temperature characteristics .DELTA.Vz of the Zener diode 104 are determined by the current Iz running through the Zener diode 104.
The temperature characteristics of the constant voltage circuit of the prior art will be described hereinunder.
The current Iz running through the Zener diode 104 is expressed by the following equation (2). EQU Iz=(V6-Vz)/R13 (2)
When the temperature characteristics .DELTA.Vz of the Zener diode 104 which are determined by the current Iz running through the Zener diode 104 at a certain temperature range are taken into consideration, the voltage V_ of the inversion input terminal of the differential amplifier 100 is expressed by the equation V_=Vz+.DELTA.Vz. Therefore, the constant voltage V6 is expressed by the following equation (3) when the temperature characteristics .DELTA.Vz of the Zener diode 104 are taken into consideration. EQU V6={(R11+R12)/R12}.multidot.(Vz+.DELTA.Vz) (3)
The temperature characteristics .DELTA.V6 of the constant voltage V6 are expressed by the following equation (4). EQU .DELTA.V6={(R11+R12)/R12}.multidot..DELTA.Vz (4)
Therefore, it is understood that the temperature characteristics .DELTA.V6 of the constant voltage V6 are proportional to the temperature characteristics .DELTA.Vz of the Zener diode 104.